1. Field of the Invention
This invention relates to FDM (frequency division multiplexed) subscriber carrier systems used in telecommunications and, more particularly, to analog terminals which automatically adapt to various changes in cable characteristics. Further, the gain and slope correction for both directions of transmission are obtained by cross-regulation as the pilot signals are only transmitted in the high frequency band.
2. Background Description
The carrier modulation employed in analog carrier systems is typically double sideband (DSB-AM) or single sideband (SSB-AM). Single-sideband modulation occupies far less bandwidth than DSB-AM for the same number of channels, but it has the disadvantage of being more sensitive to changes in gain or attenuation versus frequency (i.e. slope) in the transmission path. Fixed equalizers have often been used to compensate for this (frequency roll-off) slope characteristic, and most are manually adjustable so that cable length variations in spacing between end terminals may be accommodated. In addition to the standard variation of attenuation with frequency, there are changes in the attenuation characteristics of insulated cables as a result of temperature and weather conditions. The latter is important in cable carrier systems, such as used in telephony subscriber loops, because of the effect of moisture on cable shunt conductance. Some means of automatic level regulation is necessary to maintain the transmission levels of such systems at or near the desired values for each SSB channel.
If the overall gain were the same for each frequency, (i.e., no slope) a variable gain amplifier would be sufficient to compensate for the gain variation. Unfortunately, the frequency slope of the commonly used transmission cables also changes with variations in ambient conditions. To compensate for variable slope requires that the slope equalization characteristic also be varied for the overall gain versus frequency characteristic to remain flat.
In many prior art carrier systems, two pilots, one near each edge of the transmission band, have been used in each direction of transmission to provide a basis for both gain and slope correction. In subscriber carrier systems a form of cross-regulation has been employed. Only two pilots are transmitted; both in the high frequency band. The high frequency band is normally transmitted from the central office to the subscriber station or end terminal. This results in a relatively simple gain and slope regulator for this direction of transmission. Usually a closed loop regulator in conjunction with a pilot pick off filter are all that are necessary. The gain is adjusted until the low frequency pilot matches a predetermined level, and then the slope is varied until both pilots are at the same level.
For the return path (subscriber station to central office) the frequency band is pre-equalized. The amount of gain and slope correction that is needed must be based upon the level of the incoming (high frequency path) pilots. With minor variations the gain and slope settings are often adjusted to the same values as those in the other direction. Such an adjustment is a compromise and thus errors are introduced. Some of these errors may be manually adjusted during initial alignment using switched equalizer networks which are included for this purpose. However, the accuracy of regulation is poor particularly when changes in temperature are considered. The frequency response should be flat to within .+-.0.25 dB for each channel for cable losses of 35 dB .+-.5 dB and for cable gauges of 19, 22, 24 and 26 which are operating in ambient temperature conditions of from -40.degree. C. to +65.degree. C. Further, no manual adjustment should be necessary. It is an object of this invention to provide a subscriber carrier system having improved gain stability over a wide range of ambient temperatures.
Prior art carrier systems also have the disadvantage that if one or both system pilots fail, the gain and slope amplifier settings often are increased to the maximum possible levels which usually interrupts transmission. It is a further object of this invention to provide a subscriber carrier system where the gain and slope amplifier settings are stored in memory, so that system pilot failures will not result in an interruption of transmission.